Iron

Iron is the metal that built civilisation. It is the most widely used metal in the world, the main component of steel, and the metal that carries oxygen through every drop of blood in your body. It is also the most abundant element in the entire Earth by mass: the planet's core is mostly iron.

  • Atomic Number2626 protons, 26 electrons
  • Atomic Mass55.84 uAbout 56× heavier than hydrogen
  • State at Room TempSolidstrong, magnetic metal
  • Density7.874 g/cm³About 7.9 g/cm³, dense and heavy
  • Melting / Boiling1537.8°C / 2860.8°CMelts at 1,538°C
  • DiscoveredAncientKnown since ancient times

Iron is the reference point for atomic mass, how do nearby elements compare?

Iron's atomic mass of 55.8 u sits near the peak of nuclear binding energy, making it the most stable heavy nucleus.

Atomic Mass Comparison
Chromium52.0 u
Manganese54.9 u
Iron55.8 u
Cobalt58.9 u
Nickel58.7 u

Iron at 55.8 atomic mass units sits right at the peak of nuclear binding energy. Atoms lighter than iron release energy by fusing together; atoms heavier than iron release energy by splitting apart. Iron is the end point of stellar nucleosynthesis.

What is iron?

Iron is a transition metal in Group 8 of the periodic table. It has 26 protons and is one of only four elements that are naturally magnetic at room temperature. Iron is a hard, grey-silver metal that is very strong but rusts relatively quickly when exposed to air and water, forming reddish iron oxide (rust) that flakes off, exposing fresh metal beneath. Iron has two common oxidation states: +2 (ferrous, forming green solutions) and +3 (ferric, forming orange-brown solutions and rust).

Iron gets its name from the Anglo-Saxon word iren or isern. Its chemical symbol Fe comes from the Latin word ferrum, which is the root of "ferrous" and "ferric". The word "iron" appears in many European languages in similar forms. Iron has been known and worked by humans for at least 3,500 years: the Iron Age began around 1200 BCE, when iron smelting spread widely across the Near East and Europe, replacing bronze as the preferred metal for tools and weapons.

Fact Iron is the most abundant element in the Earth by mass, approx. 32% of the entire planet is iron, mostly in the liquid iron-nickel outer core and solid iron-nickel inner core. The Earth's magnetic field, which protects life from the solar wind and makes compasses work, is generated by swirling currents in this liquid iron outer core.

Where you find iron

In space

Iron is produced at the very end of a massive star's life. Stellar fusion builds elements up from hydrogen to helium to carbon to oxygen and beyond, but the chain ends at iron. Iron has the most stable nucleus of any element, so fusing iron atoms together releases no energy at all. When a giant star's core becomes iron, fusion stops, the star collapses catastrophically and explodes as a supernova. The Sun contains iron. Mars's red colour comes from iron oxide (rust) covering its surface.

On Earth

Iron is the fourth most abundant element in the Earth's crust, behind oxygen, silicon and aluminium, but the most abundant element in the planet as a whole.

  • Haematite (Fe₂O₃) and magnetite (Fe₃O₄). These iron oxide ores are mined on a colossal scale. Australia and Brazil together supply over half the world's iron ore.
  • Iron pyrite (fool's gold, FeS₂). Not a useful iron ore but extraordinarily common in rocks worldwide. Its metallic gold glint has famously misled prospectors throughout history.
  • Siderite and limonite. Other iron-containing minerals found in sedimentary and metamorphic rocks.

How we use iron

  • Steel. Iron alloyed with a small amount of carbon becomes steel, far stronger and more versatile than pure iron. Steel is the most widely produced metal on Earth at approx. 2 billion tonnes per year, used in everything from skyscrapers to cutlery.
  • Cast iron. Iron with 2 to 4% carbon is cast iron, hard and rigid, ideal for engine blocks, manhole covers, cookware and Victorian ironwork.
  • Haemoglobin. The iron atom in the centre of each haemoglobin molecule in red blood cells binds to oxygen in the lungs and carries it throughout the body. Without iron, blood cannot transport oxygen.
  • Magnets. Iron is strongly magnetic and is the core material in electromagnets, electric motors, generators, and transformers.
Did you know? About 70% of the iron in your body is in your blood, inside haemoglobin molecules in red blood cells. Iron gives blood its red colour, oxygenated blood is bright red, deoxygenated blood is a darker red-brown. The average adult has approx. 4-5 grams of iron in their body, most of it in the blood. Women need more dietary iron than men because of monthly blood loss.

How it was discovered

Iron has been known since prehistory, small amounts of pure iron reached Earth in meteorites, and people made tools from "sky iron" long before learning to smelt it. The earliest iron smelting dates to around 1200 BCE in Anatolia (modern Turkey) and the Middle East. The discovery that iron ore could be heated with charcoal to extract the metal was one of the most important technological advances in human history, initiating the Iron Age and transforming warfare, agriculture and construction.

Cast iron was produced in China as early as the 5th century BCE, nearly 2,000 years before Europe achieved the same temperatures. The Industrial Revolution in Europe from the 18th century onwards was fundamentally built on iron, iron railways, iron bridges, iron machines and later steel.

Deeper dive: iron in biology and the Bessemer process

Iron is essential to almost all living organisms. In humans, iron-containing haemoglobin carries oxygen in red blood cells; iron-containing myoglobin stores oxygen in muscle; and iron-containing cytochrome enzymes drive the energy-producing reactions in mitochondria. Iron deficiency anaemia: the most common nutritional deficiency in the world, causes fatigue, pale skin and shortness of breath because not enough oxygen reaches the tissues.

The Bessemer process, invented by Henry Bessemer in 1856, transformed steel making. Previously, steel was produced in small batches at enormous cost. Bessemer invented a converter, a large pear-shaped vessel, that blasted cold air through molten pig iron, burning off excess carbon rapidly through the heat of combustion alone, with no fuel required. In 20 minutes, it produced as much steel as a previous furnace managed in 24 hours. Steel prices fell by 80% within a decade, enabling the construction of iron bridges, railways and buildings on a previously impossible scale.

Iron's position at the peak of nuclear binding energy means it is literally the most stable nucleus possible. In nuclear fusion, lighter elements fuse to release energy, climbing up the periodic table towards iron. In nuclear fission, heavier elements split to release energy, falling down the table towards iron. Iron is the thermodynamic endpoint, it cannot release energy by either process. For this reason, when a massive star runs out of fuel and its core becomes iron, the star faces certain, violent death.

Iron is the metal that shaped human civilisation and carries oxygen through your blood. Moving to 27 protons brings us to cobalt, the element behind the striking blue of medieval glass and a vital component of modern batteries.